Dedicated IPs are prepared in limited numbers. If none can be assigned at the moment, Settings › Plan shows No IP available right now.
Click Notify me. Your request is saved and you get an email at your account address when an IP becomes available. You are charged only when an IP is assigned to you; nothing is paid while waiting.
Your sending does not stop while you wait: your domains keep sending from the shared IP.
When you hear from us, go back to Settings › Plan and complete the purchase with Add a dedicated IP.